fix: make sure scaladocs are correct for plugin options

This just makes a small update to the scaladocs to align what actually
happens with what the comments say. Currently the comments are incorrect
as passing in `-P:plugname:opt1,opt2` becomes List(opt1, opt2)` does not
become `List(opt1, opt2)`.

refs #17238

Author: ckipp01

compiler/src/dotty/tools/dotc/plugins/Plugin.scala

@@ -44,7 +44,7 @@ sealed trait Plugin {
 trait StandardPlugin extends Plugin {
   /** Non-research plugins should override this method to return the phases
    *
-   *  @param options commandline options to the plugin, `-P:plugname:opt1,opt2` becomes List(opt1, opt2)
+   *  @param options commandline options to the plugin.
    *  @return a list of phases to be added to the phase plan
    */
   def init(options: List[String]): List[PluginPhase]

compiler/src/dotty/tools/dotc/plugins/Plugins.scala

@@ -116,10 +116,8 @@ trait Plugins {
 
   /** Add plugin phases to phase plan */
   def addPluginPhases(plan: List[List[Phase]])(using Context): List[List[Phase]] = {
-    // plugin-specific options.
-    // The user writes `-P:plugname:opt1,opt2`, but the plugin sees `List(opt1, opt2)`.
     def options(plugin: Plugin): List[String] = {
-      def namec = plugin.name + ":"
+      val namec = plugin.name + ":"
       ctx.settings.pluginOptions.value filter (_ startsWith namec) map (_ stripPrefix namec)
     }